Formulating and packaging microwavable foods

Claudia D. O’Donnell

It’s a brand new ballgame for developers of microwavable foods. Consumers demand better product quality and enhanced convenience, but aren’t willing to pay more.

Not too long ago, food marketers could prominently display the word “Microwavable” with an artsy wave symbol on food packages, and consumers would buy nearly everything in sight. The craze for convenience and fast preparation times spelled fast bucks for food companies.

But things changed. The recession hit. Many microwavable foods became pricey in the minds of most consumers. This price/value relationship compounded the problems of product quality. And, consumers became unwilling to trade good taste and eating qualities for time saved in cooking.

Under these new rules, today’s microwavable foods must deliver more… more and better flavor, improved eye appeal, more convenient packaging, and a real point-of-difference in the marketplace.

Although consumers demand more, they’re not willing to pay more. Consequently, food formulators and package designers of microwavable foods are faced with many challenges. The development teams must weigh the technological advances of packaging systems and functional ingredients against the price constraints of the market. It’s a balancing act that, like a man on a tightrope, affords a razor thin margin of error.


Microwave ovens convert electrical energy into an electromagnetic field. This changes the direction of the positive and negative charge billions of times per second. Heat is created when dipolar molecules (such as water) rotate back and forth aligning themselves with the electric field or when the ions (such as salt which dissociates to Na+ and Cl-) migrate in response to the field.

How fast a food heats depends heavily on characteristics which can be categorized as: its thermal properties (specific heat capacity and thermal conductivity); its mechanical properties (density and viscosity); and its dielectric properties. Dielectric properties include a food’s dielectric constant, dielectric loss factor, and dissipation factor. However, “the dissipation factor is not used as much today, particularly in the food industry,” says Charles Buffler of the Microwave Research Center. The terms may be defined as follows:

* Dielectric constant is a measure of a material’s ability to reflect microwave energy. Distilled water has a dielectric constant of 78; ice is 3.2.

* Dielectric loss factor is a measure of a material’s ability to convert absorbed energy to heat. “Distilled water has a loss factor of 10.6 at 20 [degrees] C; ice is 0.003 at 0 [degrees] C,” says Buffler. “Some confusion on values exists because properties are extremely temperature dependent.”

* Specific heat is the amount of energy (calories) needed to heat 1.0 g of food to 1.0 [degrees] C. The same amount of heat will raise the temperature of a gram of oil twice as high as a gram of water.

* The depth of penetration (dp) is the depth to which microwave energy reaches until it is 37% of its value (just inside the food’s surface). Occasionally, the fraction 50% is used instead of 37%.


What does this all mean for food formulators?

When meals or foods–such as pies or sandwiches–consist of several components, over- and underheating is reduced if the component’s heating characteristics are similar. Food maps, which relate a foods’ dielectric properties, help to match components with similar microwave depths of penetration (dp).

A common problem is food surface overhearing which occurs when a food has a short dp. Lowering the level of salt–or flavorings with high sodium contents–decreases a food’s dielectric loss factor (ability to absorb energy). Microwaves penetrate deeper and the food heats slower (but possibly more uniformly). Dps also increase when water is decreased.

The lower a food’s specific heat capacity, the more quickly it will heat. Foods or components with high fat and sugar–and low water–contents have low specific heats. Examples include syrups, high-fat sausages, bakery fillings, etc. Reducing the solids or fat content of fast heating materials or surrounding fast heating components with “heat-absorbing” sauce can ease problems.

Although the creation of high quality microwave products depends heavily on the talents of product developers, suppliers contribute with improved ingredients.

In baked goods, for example, quick gelatinizing starches respond well to quick microwave heating. Instantized, cold-water swelling, and other modified or granular starches have been developed to sustain the structure of a cake or muffin to reduce the risk of collapse into a sodden mass.

Perhaps, microwavable food’s greatest challenge is a reputation for “ho-hum” flavors. In baking or broiling, high surface temperatures create conditions for the development of desirable reaction flavors. The surface of microwaved foods remains much cooler because the surrounding air does not heat. Also, quickly evaporating water cools the surface of microwaved foods.

“The first challenge is to develop flavors, and if they can’t be developed, then add them,” says Gary Reineccius, flavor specialist at the University of Minnesota. The second problem is that they “flash-off.” The problem becomes how to keep flavors with the food, he says.

Flavor encapsulation offers one approach. Flavors can be protected and then released at desired temperatures or stages through factors such as the choice of shell (or surrounding host material) and method of encapsulation.

In another development, Bruce Redding, president of Encapsulation Systems Inc., a technology licensing company, says that by encapsulating flavors with fats, starch and/or maltodextrins under three atmospheres, pressure effectively retains ketones and other volatile flavor notes.

“Despite the fact that people think the microwave [trend] is dead, I still think it has one heck of a future,” says Reineccius.


Today’s microwavable packaging appears to be heading in two opposite directions. On one hand, food manufacturers want simplified materials and designs to lower costs. For this reason, many food companies have switched to one-piece, folded paperboard packages.

Recently, Quaker Oats went to a dual-ovenable paperboard tray for its Aunt Jemima [R] coffee cake and corn bread mixes. Supplied by International Paper, the tray features an advanced gluing process that eliminates the need for doubled sidewalls, reducing material usage by approximately 15%.

On the other end of the spectrum, food manufacturers are looking for more sophistication in their microwavable packaging to improve product quality and provide them with a competitive edge in the marketplace.

“Our new Hungry Jack [R] Microwave Ready Syrup bottle stands out against the competition,” declares Ellen Perl, business team leader – breakfasts, Pillsbury Co. “It delivers a real point-of-difference in the marketplace.” Introduced in January, the product has gained a “significant presence” according to Perl.

The polypropylene bottle features several advances in microwavable packaging. Its squat shape (approximately 7-in. tall) allows it to fit inside average-sized microwave ovens. Bottle geometry also promotes uniform heating of the syrup.

Pinched at two points, the bottle’s handle is sealed to prevent product entering it. When the syrup is heated, the handle stays cool. Raised markings on the side of the 24-oz. jug divide the contents into fourths. Microwave heating instructions on the back of the container correspond to the markings.

The front label incorporates a patent-pending, heat-sensitive indicator, which resembles a microwave oven. When the syrup is heated, the oven’s “door” changes from black to white, revealing the word “HOT” in red letters. If the consumer forgets to pull up the pour spout before microwaving, it will automatically vent to release steam.


Many sophisticated microwavable packages incorporate susceptors, which are metallized structures that help heat foods more evenly. Some susceptors are designed to brown and crisp, thereby improving the appearance and texture of the food.

To improve the performance of its Qwik Crisp [R] susceptor technology, James River Corp., Cincinnati, has developed Qwik Tenna [TM], antenna-like structures with tiny loops that tune the geometry of the transmission line to the wavelength of the microwave energy.

“In essence, the antenna grabs the energy and redirects it toward the Qwik Crisp susceptor, which converts it to heat for browning the product,” explains Tim Bohrer, vice president – technology, James Rivers’ Packaging Business. “The technology is especially suited for products with a high thermal mass, such as pot pies, fruit pies and pizzas. Qwik Tenna helps to overcome the shadow effect, whereby the bottom center of a food doesn’t get all the energy it needs.”


Anchor Hocking Packaging, Cincinnati, and Owens-Brockway Glass Containers have jointly developed a microwavable glass jar with a press-on metal closure for entrees and gourmet soups and sauces.

A foamed polystyrene band wraps around the container. It serves as both a label and heat insulator. The band extends about 1/8″ over the base of the container to minimize the effects of rattling and “volcanoing” during microwaving.

The Micro-Seal [TM] closure is a combination of a non-threaded polypropylene skirt affixed to a plastisol-lined metal disk with vacuum button for tamper evidency. Prior to microwaving, the lid is loosened and left on top of the container to act as a splash guard during heating. This eliminates the need for an overcap.

The Micro-Serve [TM] jar can be hot-filled or retorted in a rotary cooker with overriding pressure. Signature Foods, division of So-Pak-Co, Mullins, S.C., has been working with the closure and jar suppliers to produce prototype test packs of products for prospective food companies.

“We have tests scheduled with a large food company in the near future,” says Bush. “This food company, who is marketing microwavable products in plastic containers, is concerned about landfilling and waste stream restrictions. The glass jar can be recycled again and again.”

Do you remember someone telling you not to put anything metal in a microwave oven? “The perception that metal containers can’t be used in microwave ovens has been a huge obstacle to overcome,” says Jack Crossett, manager of market development & technical services, Tinmill Products Div., LTV Steel.

“Steel trays have good barrier properties for shelf-stable foods; they’re dual-ovenable and can be recycled,” says Crossett. “However, the domestic market for shelf-stable foods is sluggish.”

LTV Steel has a licensing agreement with CarnaudMetalbox Food-can Group, Wantage, England, to produce its Ferrolite product, a traditional steel can laminated with polypropylene and PET. “The lamination improves the appearance of the container and protects against corrosion. It also minimizes the possibility of arcing,” says Crossett.


Efforts toward continuous improvement drive many projects; however, it was microwave oven popularity in particular that drove ConAgra to change not only their entree packaging but to some extent their formulations.

“Eighty-seven percent of the people that use Healthy Choice heat the products in a microwave oven,” says Jeff Schweid, manager of product development at ConAgra Frozen Foods. “Cook-in bags were messy–consumers indicated that trays were more convenient. Trays also were more in line with a quality frozen dinner.”

Thirteen Healthy Choice entrees were reformulated to meet consumers’ requests for better taste, but formulation changes were also needed in response to the packaging and production alterations.

For example, one of the greatest challenges was “reevaluating the types and levels of starches to adapt to the manufacturing environment,” says Schweid. There is much more evaporation from foods on a tray than in a bag. The food also dispenses differently. “In a tray there are a lot of ‘side by sides,’ with sauce positioned on one side and components on another. The sauce must be thick enough so that it doesn’t flow together with the component, even though the customer may stir them together later,” he says.

Schweid predicts future developments in microwavable food packaging will involve less packaging, more recyclability, and more reuse…if the economics allow.

COPYRIGHT 1994 Business News Publishing Co.

COPYRIGHT 2004 Gale Group

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